What Is A Healthy Plant?
What to look for when you're buying
The size of the plant should match the size of the container, not so tiny it can barely be seen, not loose, rangy and floppy. In general, younger is better. Young plants that have been hardened off properly by the grower and are ready to be moved on into their final home are always the best choice. That means plants in 6-packs (also called pony packs or color packs) or up to 4-in. pots for all annuals. Some perennials are available only in 1-gallon size containers.
Many gardeners are not aware that most plant nurseries do not grow their own plants. Rather, the plants are grown in large greenhouses by wholesale nurseries, and delivered to retail nurseries, garden centers, supermarkets, etc. in large trucks, usually on a weekly basis. How long these plants sit on the shelves and how well they are cared for waiting to be sold has a lot to do with the quality of the plant. In general, the wholesaler delivers plants that have been hardened off properly and are ready to be planted that week or the next. The longer they stand on the shelf, the greater the chance that quality will suffer.
Specialized plant nurseries (those that sell only succulents, or water garden plants, or camellias, etc.) generally propagate their own stock in their own greenhouses or growing fields. The prices may be a bit higher and the numbers of plants within a given species may not be huge, but you're dealing with a hands-on professionals who have intimate knowledge and expertise in plant care. You can be assured you'll get a plant that will perform exactly as described, given proper care, and it will be worth the slight amount of additional expense.